First attempt at ripping out agnostic code into ifcopenshell.api namespace. See #1399.

src/ifcopenshell-python/ifcopenshell/api/style/add_style.py

@@ -0,0 +1,43 @@
+class Usecase:
+    def __init__(self, file, settings=None):
+        self.file = file
+        self.settings = {
+            "Name": "Name",
+            "SurfaceColour": [], # RGB
+            "DiffuseColour": [], # RGB
+            "Transparency": 0,
+            "external_definition": {
+                "Location": None,
+                "Identification": None,
+                "Name": "Name"
+            },
+        }
+        for key, value in settings.items():
+            self.settings[key] = value
+
+    def execute(self):
+        styles = [self.create_surface_style_rendering()]
+        if self.settings["external_definition"]:
+            styles.append(self.create_externally_defined_surface_style())
+        # Name is filled out because Revit treats this incorrectly as the material name
+        return self.file.createIfcSurfaceStyle(self.settings["Name"], "BOTH", styles)
+
+    def create_surface_style_rendering(self):
+        return self.file.create_entity("IfcSurfaceStyleRendering", **{
+            "SurfaceColour": self.create_colour_rgb(self.settings["SurfaceColour"]),
+            "Transparency": (self.settings["Transparency"] - 1) * -1,
+            "ReflectanceMethod": "NOTDEFINED",
+            "DiffuseColour": self.create_colour_rgb(self.settings["DiffuseColour"])
+        })
+
+    def create_externally_defined_surface_style(self):
+        self.file.create_entity(
+            "IfcExternallyDefinedSurfaceStyle", **{
+                "Location": self.settings["Location"],
+                "Identification": self.settings["Identification"],
+                "Name": self.settings["Name"],
+            }
+        )
+
+    def create_colour_rgb(self, colour):
+        return self.file.createIfcColourRgb(None, colour[0], colour[1], colour[2])